Whether you can afford the targets making a break for it, whether you can reliably take down the big threat before it recovers, whether the smaller ones dogpiling on a player is a bigger risk than the big run taking a swing, etc. all factor in.

Heh, okay so I don't know any kind of combat all that well. I never did much with a system like D&D (and the one campaign I actually was able to half attend where we used it, I was the Cleric). When it came to typical JRPGs, I was that guy obsessed with min-maxing so I often was powerful enough to wipe out all small fry or wipe out the big guy in one hit... or else there were only small fry/only big guys. Plus I've always been open to using strategy guides. ;)

Most other RPG systems I used didn't have things like "levels" or "guaranteed kills". Sometimes it was better to take out the leader first because underlings would flee or be bolstered by said boss, sometimes it was the opposite.

Sometimes the minions buff the boss, sometimes the boss buffs the minions, sometimes it's better to get a gauranteed kill on a mook, sometimes it's better to apply as much DPS as possible on the big guy.